Commercial radiation curable inkjet inks generally contain significant amounts of low molecular weight co-initiators, such as ethyl-4-dimethylaminobenzoate or N-methyl-diethanolamine. This co-initiator is used in combination with a Norrish type II-initiator to accelerate the radiation curing process. No problem arises if all of the co-initiator is consumed and built into the polymeric network. However, hydrogen transfer from the co-initiator to the Norrish type II-initiator is rarely quantitative, resulting in unreacted co-initiator. In food packaging printed upon with a radiation curable composition, this unreacted co-initiator remains mobile and if toxic will cause health risks upon being extracted into the food. Unreacted co-initiators are also known to adversely affect the physical properties of the packaging material.
One approach in solving these problems is to design co-initiators with a higher molecular weight.
JP 2000086713 (TOYO INK) discloses the use of the reaction product of an unsaturated monomer bearing (meth)acryloyl groups or vinyl ether groups having an number average molecular weight of more than 500 with a primary or secondary amine as co-initiator in radiation curable compositions. However, using this approach only co-initiators with low functionality can be obtained.
EP 434098 A (UNION CARBIDE) discloses the use of amino terminated polyoxyalkylenes as co-initiators in radiation curable compositions. The claimed polyoxyalkylenes also have a low functionality, requiring the use of large amounts of unreactive polymer in the matrix compared to low molecular weight co-initiators.
The combination of a co-initiator and an initiator in a conventional linear polymer geometry has been described by ANGIOLINI, et al. Polymeric photoinitiators based on side-chain benzoin methyl ether and tertiary amine moieties for fast UV-curable coatings. Polymers for Advanced Technologies. 1993, vol.4, no.6, p. 375-384. Although potentially interesting to reduce extractable residues, the linear geometry of the polymer increases the solution viscosity of the formulations to an undesirable level for a great number of applications with radiation curable compositions, e.g. inkjet inks and lacquers.
WO 9907746 (DSM) discloses a radiation-curable resin composition containing at least a radiation-curable resin, a photo-excitable compound and an aliphatic amine, characterized in that as amine is chosen a compound containing at least one tertiary amino group, at least one substituent of the tertiary amino group being an aliphatic chain containing at least one electron-withdrawing group, excluding the case where the aliphatic amine consists of one tertiary amine group with the aliphatic chain being a cyanoethyl group and the other two substituents of the tertiary amino group forming part of an alkyl ring with 4 or 5 carbon atoms.
In a preferred embodiment and in all of the examples, Astramol™-dendrimers, commercially available from DSM, are used as co-initiator in comparison with low molecular compounds. However, the low generation dendrimers are in fact low molecular weight compounds, while higher generations require laborious synthetic work, making them too expensive for several applications. A further derivatization of these compounds to compatibilize them with different radiation curable compositions is impossible. The same Astramol™-dendrimers are described in WO 9903930 (DSM) in combination with maleimides as initiator.
WO 0222700 (PERSTORP SPECIALTY CHEM) discloses a radiation curable dendritic oligomer or polymer, characterized in that the radiation curable dendritic oligomer or polymer normally has at least one terminal group of Formula (A):
and normally at least one terminal group of Formula (B):
wherein R1 and R2 individually are hydrogen or methyl and wherein R3 and R4 individually are alkyl, aryl, alkylaryl, arylalkyl, alkylalkoxy, arylalkoxy, said alkyl and/or said aryl optionally having one or more hydroxyl groups. The dendritic polymers are claimed to be of particular of interest for curing under air compared to conventional curable dendritic oligomers. However, these oligomeric co-initiators tend to lose their effectiveness when coupled to a polymer, which does not contain acrylates, as stated in DAVIDSON, Stephen R. Exploring the Science Technology and Applications of UV and EB-curing. LONDON, UK: SITA Technology Ltd, 1999. p. 141. and DAVIDSON, Stephen R., et al. Type II polymeric photoinitiators (polyetherimides) with built-in amine synergist. Journal of Photochemistry and Photobiology, A: Chemistry. 1995, vol.91, no.2, p. 153-163.
There is therefore a need to provide cheap, effective co-initiators suitable for radiation curable compositions for use on food packaging with these co-initiators not being extractable into food or adversely affecting the physical properties of the packaging material. The co-initiator should be easy to manufacture and should be compatible with a wide range of radiation curable compositions without causing high solution viscosity.